THE DOUBLE STOCK, ITS HISTORY AND BEHAVIOUR. 467 

 It follows : — 



1. That where only strains of the first type are grown, so that 

 intercrossing with plants of type (2) cannot take place, no doubles 

 will be obtained, whatever the method of cultivation, save by variation 

 de novo. 



2. That where strains of type (2) only are cultivated, and under 

 conditions where interbreeding with plants of type (i) does not occur, 

 a constant average proportion of doubles, slightly in excess of the 

 singles, will be automatically maintained, apart from selection or 

 special treatment. 



Such are the patent facts, easily verifiable at a minimum expenditure 

 of labour and patience. Now to consider the underlying cause, the 

 inner nature or constitution of the two kinds of singles, which is 

 accountable for their different behaviour. 



The true-breeding single we may picture as containing factors — 

 we have reason to suppose that there are at least two and that they 

 are in some way linked together — which in conjunction determine the 

 single character of the flower. Each individual has been derived from 

 a pollen grain and an ovule containing these factors. In constitution 

 each such individual is homozygous in respect of these factors. It 

 will produce in turn pollen grains and ovules each containing these 

 same factors, and all its posterity in each succeeding generation will 

 therefore be single. 



The ever-sporting single is a heterozygous individual as regards these 

 factors. We may suppose that in the first ever-sporting individual, 

 at some stage in the sequence of cell division, there occurred an irregu- 

 larity in the normal symmetrical process, resulting in the formation of 

 some cells in which these factors were lacking. These deficient cells 

 are the producers of a certain number of the ovules, and the whole 

 number (without exception so far as we know at present) of the pollen 

 grains. It follows that the offspring of such an individual will be 

 derived in two ways : sometimes from the union of an ovule containing 

 the factors for singleness with a pollen grain that does not ; sometimes 

 from an ovule from which, as well as from the pollen grain, one or more of 

 these factors is absent. In the latter case, where both the contributing 

 germ cells are without the factors for singleness, the resulting individual 

 is a double. Being sterile, the double leads us no further ; it forms a 

 dead end in the genealogical tree. In the other case, where the 

 factors for singleness are received from one side of the pedigree but 

 not from the other, a heterozygous single results ; each such single is 

 found to behave Kke its single parent and gives again a mixture of 

 singles and doubles, and so on through succeeding generations. 



The evidence upon which these conclusions are based is derived 

 from numerous experiments in cross-breeding, and has already been 

 given elsewhere.* Briefly put, it is as foUows : — 



Where a pure-breeding single of the type described above is crossed 

 with pollen from an ever-sporting single, the resulting plants are 

 See Journal of Genetics, loc. cii: 



